Aerating apparatus

ABSTRACT

Aerating apparatus for aerating liquids such as sewage sludges, is improved by providing a rotor which moves in a path on the surface of the liquid and which moves the liquid in a direction at an angle of between 90* and 180* with respect to the path of motion of the frame. Alternatively, a device of this type is improved by providing a rotor which moves in a path on the surface of the liquid the rotor axis of which is inclined at an angle of up to 90* with respect to the horizontal.

United States Patent Josef Muskat:

Dieter Klump;.1urgen Sieckmann. all of Mlchelbach, Germany Mar. 25, 1970Nov. 16. 1971 Passavant-Werke [72] Inventors [21 App]. No [22] Filed[45] Patented [73] Assignee Hutte. Germany [32] Priority Apr. 3, 1969[33] Germany [31] P19174620 [54] AERATING APPARATUS 16 Claims, 12Drawing Figs.

[52] U.S.C1..... 261/92, 261/91.261/120,210/242 [51] lnt.Cl n B0lf7/l0,B01f7/18 [50] Field ofSearch. 261/91,)2.

l References Cited UNITED STATES PATENTS 3,208,734 9/1965 Wood et a1261/91 3,320,160 5/1967 Welles. Jr. et al... 2611120 3.322.410 5/1967Ahlenius 261/92 3,462,132 8/1969 Kaelin 261/91 3,548,770 12/1970 Boutros261/91 3.561.738 2/1971 Galeano 210/242 Primary Examiner-Tim R. MilesAssistant ExaminerSteven H. Markovitz Attorney-Larson. Taylor & HindsABSTRACT: Aerating apparatus for aerating liquids such as sewagesludges, is improved by providing a rotor which moves in a path on thesurface of the liquid and which moves the liquid in a direction at anangle of between 90 and 180 with respect to the path of motion of theframe. Alternatively. a device of this type is improved by providing arotor which moves in a path on the surface of the liquid the rotor axisof which is inclined at an angle of up to 90 with respect to thehorizontal.

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INVENTORS JOSEF MUSKAT DIETER KLUMP JURGEN SIECKMANN AERATING APPARATUSBACKGROUND OF THE INVENTION This invention relates to equipment for thesurface aeration of liquids particularly in waste water treatment.

Surface aerators are utilized in biological sewage treatment for theintroduction of atmospheric oxygen into waste water to increase thebiological action of the micro-organisms decomposing the pollutedmatter. The following two types are commonly used: rotors withhorizontal axis and rotors with vertical axis. These differ in operationparticularly in view of the horizontal and vertical flow patterns theygenerate in the water. In smaller tanks, particularly when using rotorswith horizontal axis, it is possible to agitate and aerate the entiretank volume if necessary. This, however, is not economical where largerliquid surfaces are concerned. Therefore, a different method has beenapplied by supporting a rotor or rotors on a traveling carryingstructure, for instance a bridge, spanning the tank and to provide thiscarrying structure with a drive, so it can be moved across the tank andthe rotor progressively reaches any area of the tank. This, however,involves expensive piers or pilings to support and drive the carryingstructure. Nontraveling rotors, i.e. stationary rotors, in particular,turbines, are commonly supported on pontoons so they maintain the samesubmergence depth at any liquid level variation.

lt is an object of this invention to provide an apparatus for surfaceaeration applicable in liquid in any type of basin whether in walled orsimply excavated lagoons, natural ponds or lakes, river branches, etc.not requiring any preparatory work in particular any engineering orpiling, or the like; a simple design suitable for maintenance-free,continuous operation. An apparatus of the said type is particularlyintended to increase the biological self-purification of natural watersreceiving a heavy pollution load from domestic or industrial waste watersources and/or to convert existing bodies of water into waste watertreatment plants without the requirement of any engineering andconstructional work.

BRIEF SUMMARY OF THE INVENTION The foregoing and other objects whichwill be apparent to those skilled in the art are achieved according tothe present invention by providing apparatus for surface aeration of abody of liquid comprising in combination a frame adapted to be floatedin the liquid, a bladed aeration rotor rotatably mounted in said frameand having blades at least partially extending into the liquid, meansfor rotating the rotor, means secured to said frame for guiding liquidmoved by said rotor into a horizontal flow, guide means secured to saidframe to guide the frame for motion in a predetermined path on theliquid surface, and means secured to said frame for directing at least aportion ofsaid horizontal flow at an angle of between l80 and 90 withrespect to the direction of motion of said frame in said predeterminedpath. In an alternative embodiment, these objects are achieved byproviding apparatus for surface aeration of a body of liquid comprisingin combination a frame adapted to be floated in a liquid, a bladedaeration rotor rotatably mounted in said frame with its axis of rotationinclined at an angle of up to 90 with respect to the horizontal andhaving radial blades extending into the liquid, means for rotating therotor, and guide means secured to said frame to guide the frame formotion in a predetermined path on the liquid surface.

DETAILED DESCRIPTION There follows a detailed description of a preferredembodiment of the invention, together with accompanying drawings.However, it is to be understood that the detailed description andaccompanying drawings are provided solely for the purpose ofillustrating a preferred embodiment and that the invention is capable ofnumerous modifications and variations apparent to those skilled in theart without departing from the spirit and scope of the invention.

FIG. 1 is a diagrammatic plan view of a cylindrical tank incorporatingan aerating device according to the present invention;

FIG. 2 is a diagrammatic plan view of a rectilinear tank in--corporating an alternative aerating device;

FIG. 3 is a diagrammatic plan view of an aerating rotor having ahorizontal rotor axis;

FIG. 4 is a diagrammatic plan view of an alternative aerating rotorhaving a horizontal rotor axis;

FIG. 5 is a diagrammatic plan view of the rotor of FIG. 4;

FIG. 6 is a diagrammatic plan view of an aerating rotor having a rotoraxis inclined at an angle of 90 with respect to the horizontal;

FIG. 7 is a diagrammatic elevation of an aerating rotor having a rotoraxis inclined at an acute angle with respect to the horizontal;

FIG. 8 is a diagrammatic elevation, partly in section of a rotor fornonhorizontal orientation;

FIG. 9 is a diagrammatic top view of the rotor of FIG. 8;

FIG. 10, is a diagrammatic elevation of an alternative rotor fornonhorizontal orientation;

FIG. 11 is a diagrammatic plan view of a rotor-clarifier tankcombination according to the invention; and

FIG. 12 is a diagrammatic elevation of the device of FIG. 1 l.

In apparatus according to the invention, the aeration rotor is floatmounted in a frame and guided by guiding elements along a predeterminedpath of travel, and driven by the reaction of the directed current therotor generates in the water. This type of surface aerator moves as aself-propelled watercraft over the water surface and, except foranchoring of its guide elements, requires no stationary supports,shoreline supports or the like. Thus, biological waste water treatmentby forced oxygen transfer is feasible in those cases whereconventional-type sewage treatment plants cannot be built for technicalor financial reasons.

There are various ways of generating the reaction which causes thethrust. Rotors with horizontal axis have a reaction which is normal tothe axis and opposite to the horizontal flow of water generated by therotor and which can be utilized directly for the thrust. According tothe present invention, however, the direction of forward motion of therotor frame is at an angle of less than l to the direction followed bythe horizontal flow generated in order to cut down speed and to have theflow, and thereby the aeration efiect, act upon those surface areas notwithin immediate reach of the rotor. This can be achieved according tothe invention by rotating the horizontal rotor axis in a horizontalplane to form an angle with the direction of travel of the frame or byinstallation of deflecting plates which are adjusted upstream ordownstream of the rotor at an angle to the direction of flow. In bothcases the angle is preferably adjustable for velocity control.

Rotors with a vertical axis generate a vertically ascending flow and anoverall uniform'radial or tangential flow pattern not resulting indirected reaction of a practical magnitude.- I-Iowever, a reactioncausing thrust of good magnitude can be achieved by tilting the rotoraxis relative to the vertical, or by installing deflecting plates orshutters upstream or downstream of the rotor which interfere with therotational symmetry of the-flow pattern. Naturally, these various meanscan be applied in combination.

It is an advantage if the deflecting or shutter plates are constructedas baffles additionally profiled in vertical section, so the horizontalflow generated by the rotor is deflected downwardly whichprolongs theretention time of the air bubbles introduced into the water and theoxygenation efficiency is increased. It is also advantageous if thebaffle plates are spaced at such a distance from the rotor as to extendinto the zone where the liquid lifted by the rotor action drops backinto the nonnal liquid surface.

Deflecting plates, shutter plates and/or baffles are preferably attachedto one or severalof the-pontoons used to float the rotor. Particularlywhere rotors with vertical axes are concerned, the pontoon can have theshape of a circle concentric with the rotor axis and comprising one orseveral ports for the passage of a directed horizontal flow causing thereaction.

Connecting means of the rotor frame can consist of rotatable tension orcompressionproof ties connected to an anchorage which then is the centerof a circle described by the traveling rotor frame. In oblong tanks orlagoons the rotor is preferably guided like a ferry by rectilinearguiding means such as a rope or rail, etc.

A rotor with a horizontal axis can be connected to the anchorage or to aguideway by means of connecting elements, in particular, rods ofdifferent lengths connected to the frame at the rotor axis ends andpreferably length-adjustable, so the desired angle can be easily setbetween direction of travel and rotor axis. The direction of rotation ofthe rotor is preferably reversible, either in order to reverse thedirection of travel or, in case of successive runs, to alternate theaeration effect on the areas at both sides of the traveling path. Changeof traveling direction can also be achieved by changing the angle of therotor in its frame or by changing the angle of deflecting plates, orboth.

For reversal of the drive, in particular at the reversal points of astraight rotor path, it is preferable to provide switch gear such aslimit switches, light barriers, signals to receivers for wirelesscontrol, etc. changing the sense of rotation of the rotor, or the angleof the rotor or that of the deflecting plates.

If the rotor in its travel describes a closed circle, it is advantageousto connect and drive the rotor in such a way as to direct thehorizontalflow generated by the rotor towards the center of the path. This way itis possible to also aerate the zone encircled by the traveling rotor.Since the connecting means is thus subjected to only tensile stress, itis possible to use flexible elements, such as ropes, as connectingmeans. In addition, the danger of the flow damaging the shore orembankment is decreased.

As a further development of the invention, the rotor may be connected tosludge-collecting means intended for the removal of scum or bottomsludge. While in the prior art this type of equipment was alsoattachedto traveling, propelled supports like bridges, may be secured tothe rotor frame according'to the invention and may be driven by thereaction of the rotor. Thus, other complicated attachments and drivinggear can be omitted. Such collecting means preferably incorporatedeflecting and/or baffle plates.

The application of the aeration equipment according to the invention inany type of liquid body, not especially selected for it. may prove to besomewhat inefficient as it will then depend on the shape of the body ofwater whether zones can be created where the activated sludge particlescan settle out and separate themselves from the water and in particularwhether this is possible in the discharge area, so the water drawn offis free from suspended solids. Therefore, a further especiallyadvantageous embodiment of the invention, provides near the rotor path asecondary settling tank supported on pontoons and equipped withefi'luent means for clarified water. The bottom of such secondarysettling tank can be shaped like a tub. Between the bottom of the tuband the bottom of the tank or pond, inlet ports for the polluted waterto enter should be distributed at least over a portion of thecircumference of the secondary tank. Such a settling tank, in otherwords, comprises a peripheral wall surrounding a calm settling zone intowhich the water to be purified can enter at the tank bottom at the samerate as the clarified liquid leaves the tank via overflow baffles of thelike. The secondary tank is preferably rigidly connected to the rotorand participates in its motion. One configuration also provides for atranslation motion of the secondary clarifier, i.e., the latter changesits location, the sludge settling in different spots of the tank orlagoon bottom. Thus, it may be possible to dispense with abottom-scraping mechanism. In another embodiment of the invention, thepreferably cylindrical settling tank and the rotor are rotatablysupported on a mooring in the tank center. In this case a bottom scraperblade should preferably be rigidly connected to the tank wall. It is aconstructional advantage if the pontoons carrying the settling tank ortanks and the rotors are incorporated in the tank wall, and particularlyif they consist of a circular ring about the inside or outside of thewall and have a discharge trough in the upper portion and an overflowbaffle for the purified water.

It is an essential feature of efficient biological sewage treatment,i.e. an activated sludge process, that at least a certain percentage ofthe settled sludge is returned into the purification process where thereturned sludge particles under the repeated action of the rotor furtherstimulate the activity of the micro-organisms and serve as settlingnuclei for the new activated sludge flocs. In order to impart thisfeature to this embodiment of the invention to increase purificationefficiency, the device preferably includes at least one flow-deflectingplate in the suction area of the rotor in such a way as to extend thesuction effect of the rotor into the sludge-settling zone of thesecondary clarifier. Thereby part of the settled sludge gets into thesuction reach of the rotor and while being aerated is remixed with theliquid to be purified. The suction effect resulting from installation ofthe deflecting plate preferably extends adjacent to the bottom scraperblade where considerably large quantities of settled sludge arecollected in the settling tank.

With reference to FIG. 1, there is shown an aeration tank or lagoon l atleast approximating round shape. Two horizontal aeration rotors 2,2supported on the ends of their axes on a frame comprising pontoons 3,3are operating in this tank. Each rotor has a drive motor 4,4. Connectingelements 5 in the form of rods or ropes connect the rotors to astationary mooring 6 in the tank center, so that the rotors describe acircular path around the anchoring point. The left aeration rotor 2 inFIG. 1 has connecting elements 5 of unequal length and thereby is soadjusted that the angle formed by its axis and the radial connectionline between its axis center and anchoring 6 is less than Aeration rotor12 is driven in such a way that it generates a horizontal flow indicatedby arrows directed generally towards the tank center, the horizontalflow resulting in a reaction of the rotor in the opposite direction. Dueto the angular orientation of the rotor, the reaction is not exactlyradial but has a tangential component, therefore the rotor frame isdriven by the reaction in a circular path on the surface of the liquidabout the anchoring point 6 following the sense of rotation indicated byarrow A. The right aeration rotor 2 in the drawing has its rotor axisnormal to the radial connection line between axis center and mooringpoint 6. This rotor is driven in such a way that the horizontal flowgenerated by it is directed towards the outside, i.e. towards the tankwall. Within the horizontal flow zone a deflecting plate 7 is attachedby means of connecting elements 8, for instance rods, and acts like arudder deflecting tangentially at least part of the flow. The resultingreaction bearing upon the deflecting plate 7 and thereby on rotor 2'propels rotor 2 as indicated by arrow B in a circle about mooring point6. Conventional gear (not shown) may be utilized to change the settingangle of the rotor axis of rotor 2 or the angle of the deflecting. plate7 respectively. 1

It will be understood that the horizontal flow direction of both rotorsis not an angle of 180 with respect to the path of motion of the rotor,but is at an angle of between 180 and 90 with respect thereto.Accordingly, the aerated zone extends to areas of the body of liquidremote from the rotor travel path. It is preferred that the angle isbetween about and l65 to provide a substantial amount of motive reactionand to provide aeration at areas well away from the path of motion ofthe device.

In the longitudinal, rectangular aeration tank 9 illustrated by FIG. 2,the aeration rotor 10 has a horizontal axis supported on a frameincluding pontoons l1 and can be reciprocated along a straight guideelement 12 such as a tightrope. The rotor axis is parallel to the guideelement 12, so the horizontal flow generated by the rotor driven bymotor 13 is directed normally with respect to the direction of motion ofthe rotor. Downstream of the rotor, however, deflecting plates 14 arearranged directing part of the flow in longitudinal direction, so thereaction generated drives the rotor along guide element in the directionof arrow C. Limit switches 15 at the ends of the traveling rangecooperate with suitable switch elements at the rotor to effect reversalof the traveling direction either by changing the angular position ofthe deflecting plates 7.

FIG. 3 shows design details of the guide elements of an aeration rotorsimilar to rotor 2 in FIG. 1. A rotary flange or hub 16 disposed at themooring point is connected by means of length-adjustable rods 17 to thepontoons 18 supporting the ends of the rotor axis. Screws serve to fixthe length of the rods 17 and thereby the angle of the rotor. Thepontoons 18 are shaped favorably for the circular movement of the rotor.

The rotor shown in FIGS. 4 and 5 is connected to the hub 19 of theanchoring member 20 by guide elements 21 of equal length, so the rotoraxis is normal to the radius. A baffle plate 22 installed downstream therotor deflects the horizontal flow generated by the rotor towards thebottom and thereby serves to increase the retention time of the airbubbles introduced into the water. The baffle plate 22 seen from aboveis asymmetrically attached to the rotor, so it deflects the flow in thedirection of arrow D and thereby generates the reaction in the directionof rotor motion. In addition, a bottom scraper blade 23 is connected tobaffle 22 by means of rods 24. The scraper blade is also driven by therotor reaction and urges the sludge accumulated on the basin bottom 25into a sludge trough 26. Column 20 extends from the basin bottom aboveliquid level 27 and supports hub 19 for the rotor.

The rotor 28 shown in FIG. 6 has a vertical axis and is sup ported by aframe comprising carrying rods 29 and a pontoon 30. The rotor moves asindicated by arrow E sucking in a vertically ascending flow andgenerating a horizontal flow directed through the gap of a polygonalring pontoon 30 in the direction of arrow F. The rotor is supported byrods 31 on the hub 32 of anchoring point 33 and is driven by theresulting reaction in a circular motion following the directionindicated by arrow G.

FIG. 7 shows a rotor 34 also having an axis of rotation inclined to thehorizontal driven by motor 35. The rotational axis of the rotor istilted at a certain angle to the vertical, thus creating a component ofmotion on the surface of the liquid. In this case the float 36 can be aclosed ring. In the illustrated configuration, the inner walls of thefloat are additionally sloped in such a way as to form baffles 37 whichdownwardly deflect the horizontal flow generated. The rotor is connectedto an anchor point by suitable connecting members 38.

Another possibility of creating a feed component with a rotor having avertical axis is shown in FIGS. 8-10. The rotor 39 with vertical axis asillustrated in FIG. 8 (section) and FIG. 9 (top view) has one half ofits circular influent cross section blinded by plate 40. Thus the rotorsucks in the liquid only in one half (left half in the drawings) of itsinfluent cross section and is subject to a resultant unidirectionalthrust. The rotor shown in FIG. 10 has one half of its effluent crosssection blinded by plate 41 thus the water sucked in over the entireinfluent area is thrown out only to the left and the thrust on the rotoris directed to the right.

In the embodiment illustrated by FIGS. 11 and 12, the rotor 42 isattached by means of arms 51,52 to an open cylinder 43 which issupported on a circular pontoon 46. In the center of the cylinder thereis a stationary column 47 with a pivot hearing mounted to it. Cylinder43 is attached to this bearing by means of guy ropes. Rotor 42 isslanted to the tangent of cylinder 27, thus the midvertical on its axisdoes not run through the turning center 47 of the assembly. On rotationof the rotor in the direction of arrow H, the resulting feed componenttowards the circumference makes the aeration rotor and cylinder revolveabout the turning center 47 in the direction ofarrow I.

The cylinder 43 extends almost to the tank bottom 44 and surrounds acalm secondary sedimentation zone 45 where the sludge flocs settle out.The top of the circular pontoon 46 carries an overflow weir 48 throughwhich clarified water leaves the tank. This water is conducted to thecorotational center column 47 and from there via a dip pipe connectedwith a seal to the shore. At the same rate as clear water is leaving thetank, unsedimented water enters through the ring slot between the lowerpart of the cylinder 43 and the tank bottom 44.

The sense of rotation of the aeration rotor is such that the downstreamflow generated by it is outwardly directed, whereas a suction zone iscreated on the inside. By a suitable deflecting plate 49 the suctionzone 50 is downwardly extended into an area where the ring slot islocated between the lower part of the cylinder 43 and the tank bottom 44so that at this point settled sludge is sucked in through the ring slot.Then the aeration rotor 42 thoroughly mixes the sludges with the waterto be treated. A bottom scraper blade is provided with its outer end isattached to the cylinder 43 and with its inner end is rotatablysupported on center column 47. Driven by the reaction drive of the rotor42 the blade revolves together with the rotor and the cylinder and urgesthe settled sludge at the tank bottom towards the outside. The blade isarranged so as to immediately precede the zone of an aeration rotor 42,so the suction zone 50 of the rotor 42 is bound to draw in a certainquantity of sludge accumulated by the rotor.

In order to create a weight balance for the rotor 42 located at one sideof the cylinder 27, additional pontoons may be provided, for instancearms 51, 52 can be constructed as pon' toons. Of course, weight balancecan also be achieved by installing a second rotor symmetrically with therotor 2 on the other side of the cylinder 27.

What is claimed is:

1. Apparatus for surface aeration of a body of liquid comprising incombination:

a frame adapted to be floated in the liquid;

a bladed aeration rotor rotatably mounted in said frame and havingblades at least partially extending into the liquid; means for rotatingthe rotor;

means secured to said frame for guiding liquid moved by said rotor intoa horizontal flow;

guide means secured to said frame to guide the frame for motion in apredetermined path on the liquid surface; and means secured to saidframe for directing at least a major pOl'tlOl'LOf said horizontal flowat an angle in a horizontal plane of more than and less than withrespect to the direction of motion of said frame in said predeterminedpath.

2. Apparatus according to claim 1 wherein the axis of rotation of saidrotor is horizontal.

3. Apparatus according to claim 2 wherein the horizontal axis of saidrotor is at an angle of between 90 and 180 with respect to the directionof motion of said frame in said predetermined path.

4. Apparatus according to claim 2 including baffle means secured to saidframe extending into the liquid and positioned to deflect liquidmotivated by said rotor.

5. Apparatus according to claim 1 wherein the axis of said rotor isinclined at an angle of up to 90 with respect to the horizontal.

6. Apparatus according to claim 5 wherein said directing means comprisebaffle means interfering with the rotational symmetry of the flow crosssection of said rotor.

7. Apparatus according to claim 1 including second baffle means todirect said horizontal flow downwardly.

8. Apparatus according to claim 1 including a circular pontoonsupporting said rotor within the periphery of the pontoon, said pontoonincluding at least one opening for a horizontal flow of said fluid.

9. Apparatus according to claim 1 wherein said predetermined path ishelical.

10. Apparatus according to claim 1 including a secondary clarifier tankdefined by a vertical peripheral wall in the liquid and being open atthe bottom, said predetermined path being located outside of said wall.

11. Apparatus according to claim 10 wherein said rotor is secured tosaid wall and wherein said tank is secured for rotation with said rotorabout a stationary support.

12. Apparatus according to claim 11 wherein said tank is cylindricalsaid support being located centrally of said tank,

said tank including means to convey sludge outwardly from said supporttoward said rotor.

13. Apparatus for surface aeration of a body of liquid comprising incombination:

a frame adapted to be floated in a liquid;

a bladed aeration rotor rotatably mounted in said frame with its axis ofrotation inclined at an angle of substantially less than 90 with respectto the horizontal and having radial blades extending into the liquid;

means for rotating the rotor; and

guide means secured to said frame to guide the frame for motion in apredetermined path on the liquid surface.

14. Apparatus according to claim 13 including means secured to saidframe for guiding liquid moved by said rotor into a horizontal flow.

15. Apparatus according to claim 14 including means secured to saidframe to direct at least a portion of said horizontal flow at an angleof between and with respect to the direction of motion of said framealong said predetermined path.

16. Apparatus for surface aeration of a body of liquid comprising incombination:

a frame adapted to be floated in a liquid;

a bladed aeration rotor rotatably mounted in said frame with its axis ofrotation inclined at an angle of up to 90 with respect to the horizontaland having radial blades extending into the liquid;

means for rotating the rotor;

guide means secured to said frame to guide the frame for motion in apredetermined path on the liquid surface; and means secured to saidframe to direct at least a major portion of said horizontal flow at anangle in a horizontal plane of more than 90 and less than 180 withrespect to the direction of motion of said frame along saidpredetermined path.

2. Apparatus according to claim 1 wherein the axis of rotation of saidrotor is horizontal.
 3. Apparatus according to claim 2 wherein thehorizontal axis of said rotor is at an angle of between 90* and 180*with respect to the direction of motion of said frame in saidpredetermined path.
 4. Apparatus according to claim 2 including bafflemeans secured to said frame extending into the liquid and positioned todeflect liquid motivated by said rotor.
 5. Apparatus according to claim1 wherein the axis of said rotor is inclined at an angle of up to 90*with respect to the horizontal.
 6. Apparatus according to claim 5wherein said directing means comprise baffle means interfering with therotational symmetry of the flow cross section of said rotor. 7.Apparatus according to claim 1 including second baffle means to directsaid horizontal flow downwardly.
 8. Apparatus according to claim 1including a circular pontoon supporting said rotor within the peripheryof the pontoon, said pontoon including at least one opening for ahorizontal flow of said fluid.
 9. Apparatus according to claim 1 whereinsaid predetermined path is helical.
 10. Apparatus according to claim 1including a secondary clarifier tank defined by a vertical peripheralwall in the liquid and being open at the bottom, said predetermined pathbeing located outside of said wall.
 11. Apparatus according to claim 10wherein said rotor is secured to said wall and wherein said tank issecured for rotation with said rotor about a stationary support. 12.Apparatus according to claim 11 wherein said tank is cylindrical saidsupport being located centrally of said tank, said tank including meansto convey sludge outwardly from said support toward said rotor. 13.Apparatus for surface aeration of a body of liquid comprising incombination: a frame adapted to be floated in a liquid; a bladedaeration rotor rotatably mounted in said frame with its axis of rotationinclined at an angle of substantially less than 90* with respect to thehorizontal and having radial blades extending into the liquid; means forrotating the rotor; and guide means secured to said frame to guide theframe for motion in a predetermined path on the liquid surface. 14.Apparatus according to claim 13 including means secured to said framefor guiding liquid moved by said rotor into a horizontal flow. 15.Apparatus according to claim 14 including means secured to said frame todirect at least a portion of said horizontal flow at an angle of between90* and 180* with respect to the direction of motion of said frame alongsaid predetermined path.
 16. Apparatus for surface aeration of a body ofliquid comprising in combination: a frame adapted to be floated in aliquid; a bladed aeration rotor rotatably mounted in said frame with itsaxis oF rotation inclined at an angle of up to 90* with respect to thehorizontal and having radial blades extending into the liquid; means forrotating the rotor; guide means secured to said frame to guide the framefor motion in a predetermined path on the liquid surface; and meanssecured to said frame to direct at least a major portion of saidhorizontal flow at an angle in a horizontal plane of more than 90* andless than 180* with respect to the direction of motion of said framealong said predetermined path.